Erase Disk and Re-install OS X

I am preparing an iMac to be sold/given away, and I am planning to erase the hard drive and re-install OS 10.6 (it is an older iMac). I have the Mac OS X installation on USB and I am at the step where I access Disk Utility after booting from the USB.

Here is my silly question:
Which Disk do I select to erase? The one called "Macintosh HD" or the one with the manufacturers name? If I select "Macintosh HD" should I also Erase Free Space first? I notice that if I select the Manufacturer-named disk, the Erase Free Space is not available (I assume because it will erase the entire drive).

You can't erase the one with the manufactures name because you are using the recovery partition loaded on that drive. If you wish to erase the entire drive you can boot into Internet Recovery by using CMD + ALT + R on startup and wait for the globe to finish loading. This will let you erase the entire drive.

If it is a mechanic drive, select "Security Options" and do a security erase on the entire drive. Once you have done this you can then select OS X from the main menu and install it on the newly erased drive.

Staff Member

If you are booted from an external USB drive to do the install, go ahead and select the drive brand name at the top and erase from there. That way it will erase the entire disk and kill any old recovery partitions that may be on there from previous Lion and up installs.

No need to mess with erase free space unless you are concerned with someone recovering private, personal data from that area once they get the new machine.

You can't erase the one with the manufactures name because you are using the recovery partition loaded on that drive. If you wish to erase the entire drive you can boot into Internet Recovery by using CMD + ALT + R on startup and wait for the globe to finish loading. This will let you erase the entire drive.
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Just to clear up any misconceptions about this post - The OP was installing 10.6, (Mac OS X 10.6.x, aka Snow Leopard) which is too old for a Recovery partition, and likely the iMac itself is too old to be able to boot to the internet recovery.
So, booting to the Snow Leopard installer will work great (Assuming the iMac is old enough to boot with 10.6
And, erasing with the drive manufacturer's name selected will work just fine.
I always do a partition "reset" by choosing to create 1 partition - from the Partition screen. That will be close enough to a secure erase for most uses, and doesn't put a possibly older hard drive under a lot of stress like a secure erase, or a multi-pass erase.

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